The present invention relates to semiconductor devices, and in particular relates to a semiconductor device formed over a semiconductor substrate and including a memory cell for magnetically storing a data signal.
A nonvolatile semiconductor memory device is capable of holding stored data even if a power supply voltage is turned off, whereby there is no need to supply the power supply voltage in a standby state. For this reason, the nonvolatile semiconductor memory device is widely used in portable devices that are required to have low power consumption.
One of such nonvolatile semiconductor memory devices is an MRAM (Magnetic Random Access Memory) that stores data by using a magnetoresistance effect. Moreover, one of MRAMs uses a tunnel magnetoresistive element having a magnetic tunnel junction (MTJ: Magnetic Tunnel Junction).
The tunnel magnetoresistive element includes a tunnel insulating film and two ferromagnetic material layers stacked over and under the tunnel insulating film. The resistance value of the tunnel magnetoresistive element becomes the minimum when the directions of the magnetic moment of two ferromagnetic material layers are the same, and becomes the maximum when the directions are opposite to each other. The case where the resistance value of the tunnel magnetoresistive element is minimum and maximum is associated with a data signal “0” and “1”, respectively, and thus the data signals “0” and “1” can be stored. The directions of the magnetic moment of two ferromagnetic material layers of the tunnel magnetoresistive element are permanently maintained until a magnetic field in a direction opposite to a level exceeding a threshold level is applied.
The MRAM includes a plurality of tunnel magnetoresistive elements arranged in a plurality of rows and a plurality of columns, a digit line provided corresponding to each row, and a bit line provided corresponding to each column, wherein a data signal is written to a selected tunnel magnetoresistive element by causing a magnetizing current to flow through the digit line of a selected row and also causing a write current to flow in a direction corresponding to a write-data signal to the bit line of a selected column.
FIG. 22 is a diagram showing an arrangement relation among a memory cell (tunnel magnetoresistance measure), a bit line BL, and a digit line DL.
A memory cell at an intersection between a selected digit line DL and a selected bit line BL serves as a selected cell. A magnetic field concentrates on the selected cell and data is written thereto because a current flows through the selected digit line DL and the selected bit line BL.
FIG. 23 is a diagram representing switching characteristics of a TRM element. FIG. 23 shows a relation among the directions of a magnetizing current iDL and a write current iBL and the direction of the magnetic field when data is written.
A magnetic field Hx represented by the horizontal axis indicates a magnetic field H(DL) generated by the magnetizing current iDL flowing through the digit line DL. On the other hand, a magnetic field Hy represented by the vertical axis indicates a magnetic field H(BL) generated by the write current iBL flowing through the bit line BL.
With regard to the direction of the magnetic field stored in the tunnel magnetoresistive element TMR, only when a sum of the magnetic field H(DL) and H(BL) reaches the area outside an asteroid characteristic line shown in the view, new writing is carried out. That is, when a magnetic field corresponding to the area inside the asteroid characteristic line is applied, the direction of the magnetic field stored in the tunnel magnetoresistive element TMR is not updated. Accordingly, in order to update the stored data in the tunnel magnetoresistive element TMR by a write operation, a current needs to be caused to flow through both the digit line DL and the bit line BL. Here, assume that the magnetizing current iDL in one direction is caused to flow through the digit line DL, while the write current iBL in a direction corresponding to the logic (0 or 1) of a data signal is caused to flow through the bit line BL. The direction of the magnetic field which has been stored once in the tunnel magnetoresistive element TMR (i.e., the stored data) is held in a nonvolatile manner until new data writing is carried out.
Meanwhile, in the MRAM, not only a selected tunnel magnetoresistive element but also other tunnel magnetoresistive elements of the selected row and column may be disturbed by the magnetic field, resulting in the false inversion of a data signal.
As shown in FIG. 23, a disturbance in the DL1 axis occurs in a non-selected memory cell DD on the selected digit line DL. Moreover, a disturbance in the BL1 axis occurs in a non-selected memory cell DB on the selected bit line BL.
The possibility of false inversion (the probability of false inversion) of a data signal increases in proportion to the magnitude of the disturbance magnetic field which a tunnel magnetoresistive element is receiving. As the probability of the false inversion of a data signal increases, the failure rate when used as the memory device increases and the reliability decreases.
In order to solve such a problem, a segment writing method is used in the MRAM described in Japanese Patent Application Laid-Open No. 2003-45173 (Patent Document 1). In the segment writing method, since data is written simultaneously to a plurality of memory cells belonging to a segment, there is no memory cell (non-selected cell), to which the data is not written, on the digit line DL and thus the DL1 axis disturbance can be eliminated.
FIG. 24 is a diagram representing switching characteristics of the TRM element when the segment writing is carried out.
As shown in FIG. 24, the writing area is expanded by the segment writing. Furthermore, since there is no disturbance in the magnetizing current iDL, a lot of magnetizing current iDL can be caused to flow and the write current iBL can be reduced. This can reduce the current consumption as a whole.